Zyra can kill tryndamere while he is ulted.

I was just playing an ARAM and Zyra was able to kill tryndamere while under the effects of his ult about 4 times. No the ult didn't run out. He would ult and instantly die within a second of hearing the ult broadcast, but only Zyra could kill him during this time.

Match history: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2189887345/39331512?tab=overview